The present invention relates in general to a method for forming nonwoven fabrics; and, more particularly, to a method for improving the fiber throughput capacity of 2-dimensional systems for forming air-laid webs of dry fibers on a high-speed production basis; yet, wherein the web being formed is characterized by a random dispersion of essentially undamaged, uncurled, individualized fibers disposed in a controlled cross-directional profile and is substantially devoid of nits, pills, rice and other aggregated fiber masses so as to result in a web of aesthethetically pleasing appearance and increased tensile strength, irrespective of the 10 basis weight of the web.
Conventionally, materials suitable for use as disposable tissue and towel products have been formed on papermaking equipment by water-laying a wood pulp fibrous sheet at speeds exceeding 5,000 feet per minute. Following formation of the sheet, the water is removed either by drying or by a combination of pressing and drying. As water is removed during formation, surface tension forces of very great magnitude develop which press the fibers into contact with one another, resulting in overall hydrogen bonding at substantially all fiber intersections. The hydrogen bonds between fibers provide sheet strength but result in very unfavorable tactile properties and low bulk characteristics.
To improve these unfavorable properties, water-laid sheets are typically creped from the dryer roll, reforming the flat sheet into a corrugated-like structure, thereby increasing its bulk and simultaneously breaking a significant portion of the fiber bonds, thus artificially improving the tactile and absorbency properties of the material. However, creping is most effective on low (less than about 15 lbs./2800 ft..sup.2) basis weight webs. When a higher basis weight is desired, it is conventional practice to employ at least two plies of creped low basis weight paper sheets for such uses.
Conventional paper-making methods possess the inefficient attribute of initial "overbonding", which then necessitates a creping step to partially "debond" the sheet, and have extreme water requirements which create an associated water pollution problem. Still further, the essential drying procedures consume tremendous amounts of energy.
Air forming of wood pulp fibrous webs has been carried out for many years; however, the resulting webs have been used for applications where either little strength is required, such as for absorbent products--i.e., pads--or applications where a certain minimum strength is required but the tactile and absorbency properties are unimportant--i.e., various specialty papers. U.S. Pat. No. 2,447,161 to Coghill, U.S. Pat. No. 2,810,940 to Mills, and British Pat. No. 1,088,991 illustrate various air-forming techniques for such applications.
In the late 1940's and early 1950's, work by James D'A. Clark resulted in the issuance of a series of patents directed to systems employing rotor blades mounted within a cylindrical fiber "disintegrating and dispersing chamber" wherein air-suspended fibers were fed to the chamber and discharged from the chamber through a screen onto a forming wire--viz., J. D'A. Clark U.S. Pat. Nos. 2,748,429, 2,751,633 and 2,931,076. However, Clark and his associates encountered serious problems with these types of forming systems as a result of disintegration of the fibers by mechanical co-action of the rotor blades with the chamber wall and/or the screen mounted therein which caused fibers to be "rolled and formed into balls or rice which resist separation"--a phenomenon more commonly referred to today as "pilling". Additionally, J. D'A. Clark encountered problems producing a web having a uniform cross-direction profile, because the fiber input and fiber path through the rotary former was not devoid of cross flow forces.
A second type of system for forming air-laid webs of dry cellulosic fibers which has found limited commercial use has been developed by Karl Kristian Kobs Kroyer and his associates as a result of work performed in Denmark. Certain of these systems are described in: Kroyer U.S. Pat. Nos. 3,575,749 and 4,014,635; Rasmussen U.S. Pat. Nos. 3,581,706 and 3,669,778; Rasmussen et al. U.S. Pat. No. 3,769,115; Attwood et al. U.S. Pat. No. 3,976,412; Tapp U.S. Pat. No. 4,060,360; and, Hicklin et al. U.S. Pat. No. 4,074,393.
This type of sifting equipment suffers from poor productivity especially when making tissue-weight webs. For example, the rotor action concentrates most of the incoming material at the periphery of the blades where the velocity is at a maximum. Most of the sifting action is believed to take place in these peripheral zones, while other regions of the sifting screen are either covered with more slowly moving material or are bare. Thus, a large percentage of the sifting screen area is poorly utilized and the system productivity is low. Moreover, fibers and agglomerates tend to remain in the forming head for extended periods of time, especially in the lower velocity, inner regions beneath the rotor blades. This accentuates the tendency of fibers to roll up into pills.
In an effort to overcome the productivity problem of such systems, complex production systems have been devised utilizing multiple forming heads--for example, up to eight separate spaced forming heads associated with multiple hammermills and each employing two or three side-by-side rotors. The most recent sifting type systems employing on the order of eighteen, twenty or more rotors per forming head, still require up to three separate forming heads in order to operate at satisfactory production speeds--that is, the systems employ up to fifty-four to sixty, or more, separate rotors with all of the attendant complex drive systems, feed arrangements, recycling equipment and hammermill equipment.
During the 1970's a series of patents were issued to C. E. Dunning and his associates which have been assigned to the assignee of the present invention; such patents describing yet another approach to the formation of air-laid dry fiber webs. Such patents include; Dunning U.S. Pat. Nos. 3,692,622, 3,733,234 and 3,764,451; and, Dunning et al. U.S. Pat. Nos. 3,776,807 and 3,825,381. However, this system requires preparation of pre-formed rolls of fibers having high cross-directional uniformity and is not suitable for use with bulk or baled fibrous materials, such that, to date, the system has found only limited commercial application.
Indeed, heretofore it has not been believed that air-forming techniques can be advantageously used in high speed production operations to prepare cellulosic sheet material that is sufficiently thin, and yet has adequate strength, together with softness and absorency, to serve in applications such as bath tissues, facial tissues and light weight toweling.